The present invention relates to a forward-transfer DC/DC conversion circuit.
The present invention relates more particularly to a new forward-transfer converter circuit known as a xe2x80x9cforward converterxe2x80x9d.
In the field of DC/DC converters, it is known practice to use converters to transform a unipolar voltage, which is preferably a direct voltage, into another unipolar voltage, which is preferably a direct voltage, while at the same time ensuring galvanic isolation. One type of converter is known as a xe2x80x9cforwardxe2x80x9d converter.
Galvanic isolation consists in isolating the primary winding from the secondary winding of a transformer, also referred to hereinbelow as primary and secondary, respectively, and in making it possible to have a potential difference across the secondary which is different from that across the primary.
The transformer ratio is the voltage across the secondary relative to the voltage across the primary.
The xe2x80x9cforwardxe2x80x9d converter has the drawback of accumulating magnetization energy on the core of the transformer.
It is known practice in the prior art to use an additional winding placed on the core of the transformer, referred to hereinbelow as a tertiary winding or more simply a tertiary, which returns this magnetization energy to the voltage source so as to extract this magnetization energy.
Several types of xe2x80x9cforwardxe2x80x9d converter may be distinguished, in particular the one-switch forward converter, two-switch forward converter or parallel forward converter.
As regards the present invention, the one-switch forward converter forms the basis of the invention.
It has the property of being able to process very large currents, which, by actuating the switch from the open position to the closed position, make it possible to produce the non-conducting phase and the conducting phase.
The non-conducting phase serves to discharge the magnetization energy accumulated on the core of the transformer, while the conducting phase serves to convert the supply voltage.
However, in order to ensure complete demagnetization after the conducting phase, it is currently necessary for the non-conducting phase to be long enough and for it to last for at least 50% of the full cycle of the cumulative conducting and non-conducting phases.
In technical terms, this behavior is characterized by a duty cycle (time closed over total period) of less than or equal to 50%.
Two main problems arise in practice during the use of the one-switch forward converter.
First, the energy yield is reduced due to the fact that there is a conversion time, i.e. a conduction time of the switch which is shorter than the restoration or demagnetization time of the transformer, and, in addition, due to the fact that the energy contained in the leakage inductance is not recovered.
Secondly, it is necessary to oversize most of the components, and in particular the switch and the transformer, since they are subject to sudden changes in voltage or current that are very large in value.
For the one-switch xe2x80x9cforwardxe2x80x9d converters, the prior art discloses the use of diodes of xe2x80x9cclampingxe2x80x9d type and/or non-dissipative switching circuits (xe2x80x9csnubbersxe2x80x9d).
These modifications of the standard forward converter make it possible, on the one hand, to return the energy stored in the magnetization and leakage inductances of the transformer to the voltage source, and, on the other hand, to limit, to a predetermined value, of the order of twice the input direct voltage, the overvoltages across the switch.
Thus, compared with the conventional assembly, document U.S. Pat. No. 4,268,898 proposes to insert a capacitor in series between the primary and tertiary windings of the transformer. The diode conducts in the direction of return of the energy to the source via the tertiary winding of the transformer. The capacitor between the primary and tertiary windings is similarly featured in the article by Machin and Dekter (Proceedings of: xe2x80x9c19th International Telecommunications Energy Conference, INTELEC97xe2x80x9419/23 October, Melbourne, Australiaxe2x80x9d). By means of this capacitor, the diode begins to conduct once the voltage across the switch exceeds twice the input voltage.
In the assemblies disclosed in document DE-A36 34990 and in the article by Varga and Losic (Proc. 4th Annual Applied Power Electronics Conf. And Exp. (APEC), Baltimore, Mar. 13-17, 1989, pp. 40-45), it is proposed to place a second diode in series with the tertiary winding, in the same conducting direction as the primary winding. A capacitor is connected firstly between the primary winding of the transformer and the switch, and secondly between the two diodes.
As in the case of the conventional forward assembly, these variants still have the tertiary winding in the opposite direction to that of the primary winding and a cyclic ratio of less than or equal to 0.5. This is a considerable limitation with respect to the amount of energy which can be transferred to the load placed on the secondary side of the transformer.
In addition, the number of turns n3 on the tertiary winding is generally equal to the number of turns n1 on the primary winding (n3/n1=1).
Document EP-A-0 474 471 discloses a converter for converting direct voltage by zero-voltage rectangular signal switching, including an input for a source of direct voltage, a transformer comprising a primary winding and, connected to the first input, a main switch connected in series with the primary winding and the input of the source of direct voltage, as well as a clock signal generator to control the main switch. This converter comprises an auxiliary switch and a magnetic reversal capacitor connected in series with the primary winding and with the input of the source of direct voltage, this auxiliary switch being controlled by the clock signal generator in phase opposition relative to the main switch, and serves to reset the transformer. It also includes switching delay means.
The aim of the present invention is to obtain an improvement in the conventional one-switch forward converter electronic circuit, while avoiding the drawbacks of the prior art.
Another aim of the present invention is to propose better exploitation of the electronic components which constitute the body of the electronic circuit and to distribute the energy better throughout all of the said components.
In particular, one aim of the present invention is to increase the power density both per unit volume and weight and per unit cost of the various components.
Another additional aim of the present invention is to recover the energy from the leakage and magnetization inductances of the transformer by a part of the circuit during the operating phases of the converter.
Another additional aim of the present invention is to improve the dynamic behaviour of the converter and to increase its handling range.
The present invention relates to a DC/DC converter controlled by a rectangular signal, comprising an input for a source of direct voltage, a transformer comprising a primary winding connected to the source of direct voltage and a secondary winding connected to the load, a single switch in series with the primary winding and the input of the source of direct voltage intended to produce the rectangular signal, as well as a tertiary winding located close to the primary and secondary windings on the core of the transformer.
The tertiary winding is preferably in series with a diode on a branch which is in parallel to the branch comprising the primary winding and the switch.
According to a first characteristic of the invention, the tertiary winding on the transformer has the same direction of winding as the primary winding.
The tertiary winding may be weakly coupled, optionally wound on a separate core.
According to a second characteristic of the invention, the converter comprises electronic components for making up a first current flow circuit to remove at least the energy contained in the leakage inductance and in the magnetization inductance of the transformer, and a second current flow circuit to return this accumulated energy in the form of a reverse magnetization induction to the transformer.
With this aim, a magnetic reversal capacitor for resetting the transformer is provided, preferably placed between the branch comprising the primary winding and the switch and the branch comprising the tertiary winding and the diode, as well as a second diode which is in the blocking direction with respect to the source of direct voltage in series with the tertiary winding.
The first current flow circuit thus comprises the primary winding, the capacitor and the first diode for forming a current flow loop through which flows a current from the primary winding to the capacitor, making it possible to accumulate at least the leakage and magnetization inductance energy of the transformer.
The second current flow circuit comprises the primary winding, the capacitor, the second diode and the tertiary winding for forming a current flow loop with the source of input voltage from the converter, through which loop flows a current from the capacitor to the primary winding to return the energy accumulated on the capacitor to the transformer in the form of a reverse magnetization induction.
The converter according to the present invention advantageously has improved dynamic behavior and improved energy behavior by means of positioning the tertiary winding in the same direction of winding as the primary and secondary windings of the transformer, and by means of adding only two additional components, namely a capacitor and a second diode.
The type of behaviour of the converter is mainly determined by the choice of these two elements and their arrangement in the circuit to lead to the formation of the two abovementioned current flow circuits.
A rectangular signal is obtained from the voltage source by placing the switch cyclically in a conducting phase and in a non-conducting phase. This rectangular signal switching can be carried out in particular at constant frequency by means of a conventional control circuit.
When the switch is a transistor, the control circuit serves to give a control signal of low electrical potential in order to place the transistor in forward phase or in conducting mode.
The absence of this control signal results in blocking of the transistor and thus the non-conducting mode.
During passage from the conducting phase to the non-conducting phase of the switch, the converter circuit first absorbs the energy contained in the transformer by means of a current flow loop comprising the primary winding of the transformer, the capacitor and the first diode. The direction of flow in this loop is determined by the first diode, which will conduct only for a current flowing from the primary winding to the capacitor.
Thus, the energy absorbed at the transformer is stored in the capacitor.
Next, when the magnetization energy is completely removed from the transformer, the converter circuit returns the energy stored in the capacitor in the form of a reverse magnetization induction to the core of the transformer, with the aid of a second current flow circuit comprising in series the primary winding, the capacitor, the second diode and the tertiary winding, these elements forming a loop with the source of input voltage of the converter.
The direction of flow in this loop is determined by the second diode, which will conduct only for a current flowing from the tertiary winding to the capacitor and from the capacitor to the primary winding.
Thus, the energy stored in the capacitor is transferred via the primary and tertiary windings in the form of a reverse magnetization induction to the core of the transformer.
The magnetization and leakage inductances are thus prevented from leading to additional energy losses by first recovering this energy in the converter circuit and then returning this energy to the transformer in the form of a reverse magnetization induction.
An additional characteristic of the present invention relates to the fact that the magnetization current through the transformer, which, due to the fact that it has been reversed during the return of the energy stored in the circuit to the core of the transformer, has a virtually symmetrical bipolar appearance compared with the value 0 for complete demagnetization of the transformer core.
Specifically, the core of the transformer undergoes a positive and negative magnetization induction according to a balanced distribution compared with the completely demagnetized situation.
This results in better use of the core, the volume of which may be reduced by a half or even more, when compared with the cores used in conventional one-switch forward converters.
Another particular advantage of the present invention relates to the conducting period of the switch.
Specifically, as a result of better distribution and use of the transformer""s magnetization and leakage energy, this transformer demagnetizes faster, a consequence of which is that the demagnetization time of the core or the non-conducting time of the switch may be shorter than the conducting time of the switch. This phenomenon is characterized by a duty cycle of greater than 50%.
This results in better exploitation of the transformer, whose current DC/DC conversion time is longer than its demagnetization time.
Another advantage of the present invention lies in the fact that the switch produces soft switching due to the fact that sudden variations in voltage across the switch are excluded.
Most particularly, during passage from the conducting phase to the non-conducting phase, the switch cuts the flow of current from the source of input voltage to the primary winding. However, the flow of current in the secondary winding is not thereby cancelled and the secondary winding continues to pass a current which is finally reflected onto the primary winding.
To prevent the image of this secondary current on the primary winding from suddenly increasing the voltage across the switch, the electronic circuit of the converter according to the present invention provides for the image of the secondary current to flow through the capacitor in the first flow circuit.
Thus, the first diode and the capacitor produce a switching circuit, also known as a non-dissipative xe2x80x9csnubberxe2x80x9d, which allows the switch to open at zero voltage and with soft switching.
In addition, the fact that the capacitor is placed in series with the primary winding brings about damping of any parasitic oscillations, thus making it possible to reduce the size and complexity of any filtering circuits.
Another advantage of the converter according to the present invention lies in the fact that the maximum voltage across the switch S remains constant. Specifically, the maximum voltage across the switch S is proportional both to the input voltage (VDC) and to the duty cycle (D). The maximum stress is thus proportional to the product VDCxc3x97D, i.e. to the output voltage, which is often regulated and thus constant.
Considering that the duty cycle D can take a value greater than 50%, a wide range of values is obtained. In particular, the supply voltage VDC at the converter input can readily be chosen within a range extending from once to twice the value and which is, for example, between 40 volts and 80 volts or even more, without, however, having to use components of larger value or size for the core of the transformer and the switch.
The duty cycle may thus be chosen within a range from 0 to 100%.
Finally, the converter according to the present invention especially has flexible dynamic behavior, in particular in passing from one mode of stationary functioning to another mode of stationary functioning by simply varying the duty cycle or the input voltage or the like. The converter then rapidly finds a stationary regime without system runaway.
In general, the combination of all the advantages mentioned above (while at the same time maintaining a very simple control with only one switch) is certainly not encountered in the assemblies known to date.
In addition, while it is accepted that, according to the prior art, switching circuits, protective circuits and filter circuits are required for correct functioning, the converter according to the present invention comprises overall fewer electronic components than those already known.
According to another preferred embodiment, a third diode is provided in parallel on the tertiary winding and the second diode of the second flow circuit.
Preferably also, a quaternary winding is provided, arranged in series with the third diode.
Other characteristics and advantages of the invention will become apparent on reading the description illustrating one preferred embodiment of the invention.